1. Field of the Invention
The present invention relates to a low emissivity film which is excellent in moisture resistance.
2. Discussion of Background
A film composed of (2n+1) layers (n.gtoreq.1l) such as a film composed of three layers in which an oxide film, an Ag film, and an oxide film are successively coated on a surface of a substrate, or a film composed of five layers in which an oxide film, an Ag film, an oxide film, an Ag film and an oxide film are successively coated on a surface of a substrate, is a heat mirror film called Low-E (Low-Emissivity) film. A glass in which such a low-E film is formed, is called a Low-E glass.
This glass is a functional glass which can prevent lowering of room temperature by reflecting the thermal infrared radiation emitted from within a heated room, which is mainly used in cold district for the purpose of decreasing heating load. Furthermore, since this glass has a heat insulating effect of the solar radiation energy, it is adopted in a windshield of an automobile. Since this glass is transparent and is electrically conductive, it has a utility as an electromagnetic shielding glass. When this glass is equipped with an electric heating means such as a bus bar composed of an electrically conductive printing or the like, this glass can be used as an electrically heated window.
As a major Low-E glass, one having a film composition of ZnO/Ag/ZnO/glass, is pointed out. However, since such a film is devoid of durability such as scratch resistance or chemical stability, it cannot be employed in a form of single plate and it is necessary to employ it in a form of double glazing or laminated glass. Especially it has a problem in moisture resistance, wherein white dot or white turbidity is caused by moisture in the air or by moisture contained in an intermediate film in case of the laminated glass. Therefore, caution is required in the storage or in handling thereof in the form of single plate.
A detailed investigation on the deteriorated portion of the conventional Low-E glass (film composition: ZnO/Ag/ZnO/glass) reveals that damage of the film such as wrinkle, or crack, or exfoliation takes place on the outermost layer of the oxide film. Furthermore, it is found that considerable grain growth of Ag takes place. From these facts, the mechanism of the generation of the white turbidity is found to be as follows. The outermost layer of the oxide film can not resist with internal stress of the film, is exfoliated from an interface with an Ag film, and is destroyed. Next the grain size of Ag increases. Light is scattered by the destroyed surface and the large Ag grain, which causes the white turbidity in appearance.
Therefore, the inventors found that decreasing the internal stress of the oxide film, as a measure for improving the moisture resistance, is extremely effective. (described in Japanese Patent Application No. 191063/1991) By this measure, the durability of the film is considerably improved. However, it is necessary to further enhance it.